PROJECT SUMMARY The ability to correlate between large-scale developmental milestones and micro-scale cellular and protein- specific changes is a significant unmet need in the study of developmental biology. The overall objective of this work is to develop a multi-modality imaging platform that can provide time resolved three-dimensional images of tissue development, with high temporal and spatial resolutions at a molecular level. Current studies rely on multiple imaging modalities to collect information on critical stages of vertebrate embryogenesis, and most offer only static snapshots of a single developmental stage. Live imaging with optical coherence tomography (OCT) provides high temporal resolution with contrast between tissue structures, allowing researchers to identify and test the mechanisms underlying developmental processes. Three- dimensional fluorescence approaches such as confocal and light sheet microscopy (LSM) provide increased resolution and molecular specificity which can be used to observe cellular mechanisms, such as the presence of erythroblasts indicating active blood flow, that are inaccessible to lower-resolution techniques. This will be accomplished by designing and developing a novel microscopic imaging system that provides spatially and temporally aligned OCT and light sheet microscopy images. Simultaneous images will be collected through OCT scanning and fluorescent light sheet excitation of the same sample plane. Fluorescence emission will be imaged through a second objective, while the OCT signal will be collected through the same lens in reflection mode. Software will be designed to synchronize data collection with an integrated high-precision rotational stage. A novel software toolkit will be developed to analyze this rich multi-modal data. Novel reconstruction methods will be designed to fuse both modalities, while addressing the sparse and multiplex nature of the LSM images and high frame rate of OCT. Finally, we'll use this tool to test the central hypothesis that a combined LSM+OCT imaging system can reveal the precise structural and molecular events required to form a circulatory loop between the embryo and maternal chorio-allantoic placenta. Successful accomplishment of the proposed work will generate a novel, integrated imaging platform, including instrumentation and analytical software, which could be widely adopted by developmental biologists to bridge the gap between large-scale developing phenotypes and the underlying molecular and cellular processes. We will benchmark this accomplishment by identifying currently unknown critical milestones in murine embryonic development. Specifically, LSM+OCT will be used to define the precise series of events necessary to form the umbilical artery (UA) and umbilical vein (UV). This research will clarify the sequence of events, including cellular, molecular, and global phenotypic changes, that lead to the establishment of an embryonic circulatory system between the mother and developing fetus, a critical prerequisite for embryonic survival.